The pressure exerted by diffusing particles is called diffusion pressure.

Diffusion pressure is directly proportional to the concentration of the diffusing particles.

The diffusion pressure of pure solvent is always greater than the diffusion pressure of solutions.

DPD: Diffusion pressure deficit

A pure solvent has maximum diffusion pressure.

By adding solute particles in solvent its diffusion pressure is lowered.

This pressure by which diffusion pressure of a solution is lower than that of its solvent is called diffusion pressure deficit.

"The amount by which diffusion pressure of a solution is lower than that of its pure solvent is known as diffusion pressure deficit."

It was described as suction pressure by Renner (1915).

But the recent trend is to use the term water potential to explain diffusion of water.

DPD of solution = DP of solvent – DP of solution

3. OSMOSIS

Definition

Osmosis is a special type of diffusion of liquids.

When two solutions of different concentrations are separated by a selectively permeable membrane, diffusion of water or solvent molecules takes place from the solution of lower concentration to the solution of higher concentration. This process is called osmosis.

How osmosis works....?

A. PERMEABILITY

The ability of the entry and exit of water into and out of the plant cells is due to a phenomenon called permeability of the plasma membrane.

Based on permeability:

B. TYPES OF SOLUTION BASED ON CONCENTRATION DIFFERENCE

Demonstration of osmosis:The process of osmosis may be demonstrated by the simple potato osmoscope.Potato Osmoscope

Demonstration of osmosis in a living system can be done using the potato osmoscope.

A potato is peeled and one side is flattened which serves as the base.

A cavity is made in the potato and is filled with concentrated sugar solution and a pin mark is made to indicate the initial level.

This potato is then placed in a beaker containing coloured water for some time.

Observation

It is observed that the sugar solution in the cavity of the potato becomes coloured and level rises.

Inference

This proves the entry of water into the sugar solution through the potato tissues which serve as the selectively permeable membrane.

Experimental set up

After experiment

Some other methods of osmosis demonstration:

1. Osmosis by thistle funnel

2. Osmosis by egg osmoscope

Endosmosis and exosmosisExosmosis:

When a cell is placed in a solution of higher concentration than that inside its vacuole, water diffuse out of the cell.

Cell becomes flaccid.

Endosmosis:

When a cell is placed in a solution of lower concentration than that inside its vacuole, water enters the cell.

Cell becomes turgid.

Significance of osmosis

Absorption of water by root hairs

Cell to cell movement of water

Induces turgidity helping in stretching of stem

Mechanical support in non-woody plants

Growing tips remain turgid can penetrate into soil.

Regulates opening and closing of stomata.

Resistance to drought and frost

Movement of plant parts (Mimosa pudica)

Plasmolysis

When a plant cell is placed in hypertonic solution, the process of exosmosis starts and water from the cell sap diffuses out into the solution of external medium.This causes a reduction in the tension of the cell wall and brings about the contraction of protoplasm due to the continuous loss of water.

The protoplasm becomes rounded in shape due to contraction and such a cell is said to be plasmolysed and the phenomenon is referred to as plasmolysis.

The initial stage of plasmolysis where the protoplasm just starts leaving the cell wall is called incipient plasmolysis.

Reverse plasmolysis:

When a completely plasmolysed cell is again placed in water or a hypotonic solution, endosmosis takes place and the protoplasm regains its original state and shape. i.e., the cell becomes fully turgid. This phenomenon is the reverse of plasmolysis and is called deplasmolysis.

Significance of plasmolysis:

Helps to understand the living nature of a cell.

Determines osmotic pressure of cell.

Used to prove the permeability of cell wall and selectively permeable natureof plasma membrane.

Explains process of osmosis.

Killing of weeds.

Helps to preserve meat, jellies and used in pickling as their salting kills bacteria by plasmolysis.

Osmotic Pressure

When a cell or a solution is separated by semipermeable membrane from water, pure water tends to enter the solution by osmosis.

Now the pressure required to prevent the osmotic entry of water into solution.

Osmotic pressure of a solution is the pressure which must be applied to it in order to prevent the passage of solvent due to osmosis.

In other words, it is that pressure which is needed to check the process of osmosis.

Turgor pressure

When a plant cell is placed in water, it enters the cell by osmosis.

This results in an hydrostatic pressure called the turgor pressure (TP) which exerts on the cell wall, membrane and other structure contains it .

This pressure responsible for pressing the plasma membrane against cell wall is called turgor pressure.

"Turgor pressure may also be defined as the hydrostatic pressure developed inside the cell on the cell wall due to endosmosis."

Significance of turgidity

It is essential for plants to live and grow.

Provide mechanical support to non woody parts of the plants.

Loss of turgidity leads to wilting of leaves and drooping of shoots.

Opening and closing of stomata are regulated by the turgidity of the guard cells.

Wall Pressure

As a result of turgor pressure on the cell wall, the rigid cell wall exerts an equal pressure in the opposite direction called wall pressure.

Under these conditions, the plant cell is said to be turgid.

When wall pressure becomes equal to turgor pressure, entry of water into the cell stops.

The component of water potential due to the hydrostatic pressure that is exerted on water in a cell.

Pressure potential which is denoted as Ψp is equivalent to either the wall pressure or turgor pressure.

In turgid plant cells it usually has a positive value.

In xylem cells there is a negative pressure potential, or tension, as a result of transpiration.

Differences between Diffusion pressure deficit and Water potential

SOIL WATER (gravitational, capillary, hygroscopic and combined water – only definitions)

Rain is the principal source of water for the soil.

Gravitational or free water- Water which flows down due to the force of gravity is known as gravitational water. However, it is a big soil water reservoir and is trapped out through tube wells.

Capillary water: A certain amount of rain water is retained within the intercellular spaces of the soil particles in the form of a capillary network. It is called capillary water and is used by the plants.

Hygroscopic water :Some water molecules form a thin sheet of water around soil particles. It is called hygroscopic water (water of imbibition).

Combined water: The water, which is bound up in chemicals is called combined water or crystalline water. (e.g. MgSo4.7H2O).

Holard –total amount of water present in the soil.

Chesard- amount of water available to plants.

Echard- amount of water cannot be absorbed by plants.

1. ABSORPTION OF WATER

1. Absorption of water (Fig: 2.1)

Absorption of water occurs in plants through roots.

The zone of water absorption in root is about 20 - 200mm from the root tip and this is the root hair zone.

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